One application of conventional impedance spectroscopy is to attempt to determine the concentration of glucose and other substances in body fluids. In particular, this technology is of substantial interest for the determination of glucose concentration in blood and/or inter— or intracellular liquid.
It is well known that glucose and electrolyte concentrations vary in blood and underlying tissues. Several techniques for providing glucose analysis are known. These techniques permit subjects to determine their own glucose levels. Unfortunately many such techniques require invasive sampling of the subject.
Impedance spectroscopy practitioners have attempted to determine glucose concentration noninvasively. Stated another way, impedance spectroscopy practitioners have attempted to determine glucose concentration in a body liquid by analyzing the interaction of electromagnetic waves with the target material. The goal of such a technique is to provide a non-invasive in-vivo analysis.
U.S. Pat. No. 5,792,668, incorporated herein by reference in its entirety, describes one example of a device for measuring blood level glucose. According to this patent, one brings two electrodes into direct contact with the human body and attempts to measure the impedance between the electrodes.
However, one drawback of conventional impedance spectroscopy involving the direct contact of two electrodes with the human body is that the results often depend to some extent on variables affecting the electrical contact between the body and the electrodes, such as variables describing the surface condition of the body at the point of contact. As a consequence, such techniques have limited resolution in the measurement of the concentration of blood level glucose. Thus, a need remains for methods and systems for the accurate, effective, and noninvasive determination of the concentration of a substance (such as glucose) in a target (such as a body liquid). Furthermore, a need remains for methods and systems for non-invasively determining the concentration of a substance in a target, the methods and systems being applicable in other contexts such as in environmental monitoring and in food processing.